Process for the thermal stabilization of polyacrylonitrile fibers and films

ABSTRACT

AN IMPROVED PROCESS IS PROVIDED FOR THE PRODUCTON OF STAILIZED ACRYLIC FIBERS AND FILMS. STANNOUS CHLORIDE IS INCORPORATED IN A SOLUTION OF AN ACRYLIC POLYMER PRIOR TO FORMING FROM SAID SOLUTION A FIBROUS MATERIAL OR FILM WHEREIN THE PENDANT NITRILE GROUPS PRESENT IN THE ACRYLIC POLYMER ARE SUBSTANTIALLY UNCYCLIZED, AND THE ACRYLIC FIBROUS MATERIAL OR FIL HAVING 0.5 TO 10 PERCENT BY WEIGHT STANNOUS CHLORIDE INCORPORATED THEREIN IS HEATED IN A GASEOUS ATMOSPHERE CONTINING 30 TO 100 PERCENT BY WEIGHT MOLECULAR OXYGEN UNTIL A STABILIZED FIBROUS MATERIAL OR FLM IS FORMED. THE PRESENCE IF THE STANNOUS CHLORIDE IN COMBINATION WITH THE GASEOUS ATMOSPHERE CONTAINING MORE THAN THE USUAL CONCENTRATION OF MOLECULAR OXYGEN (E.G. 10 TO ABOUT 20 PERCENT BY WEIGHT) HAS BEEN FOUND TO RESULT IN A SUBSTANTIALLY IMPROVED PROCESS. MORE SPECIFICALLY, THE RESULTING STABILIZED ACRYLIC FIBROUS MATERIALS AND FILMS EXHIBIT ENCHANCED PHYSICAL PROPERTIES (I.E. STRENGTH AND MODULUS), AND THE STABILIZATION REACTION IS ACCELERATED IN A CONTROLLED MANNER IN THE SUBSTANTIAL ABSENCE OF UNDESIRABLE FIBER COALESCENCE. THE RESULTING STABILIZED FIBROUS MATERIAL OR FILM IS NON-BURINING, AND MAY BE UTILIZED AS A FIRE RESISTANT FIBER, FABRIC OR FILM, OR OPTIONALLY CARBONIZED AND CARBONIZED AND GRAPHITIZED TO FORM A CARBONACEOUS FIBROUS MATERIAL OR FILM.

United States Patent O 3,813,219 PROCESS FOR THE THERMAL STABILIZATIONOF POLYACRYLONITRILE FIBERS AND FILMS Andrew H. Di Edwardo, Parsippany,and Klaus H. Gump, Gillette, NJ., assiguors to Celanese Corporation, NewYork, NY. No Drawing. Filed Apr. 28, 1972, Ser. No. 248,371 Int. Cl.C01b 31/07; D06c 7/04 US. Cl. 8--115.5 19 Claims ABSTRACT OF THEDISCLOSURE An improved process is provided for the production ofstabilized acrylic fibers and films. Stannous chloride is incorporatedin a solution of an acrylic polymer prior to forming from said solutiona fibrous material or film wherein the pendant nitrile groups present inthe acrylic polymer are substantially uncyclized, and the acrylicfibrous material or film having 0.6 to 10 percent by weight stannouschloride incorporated therein is heated in a gaseous atmospherecontining 30 to 100 percent by weight molecular oxygen until astabilized fibrous material or film is formed. The presence of thestannous chloride in combination with the gaseous atmosphere containingmore than the usual concentration of molecular oxygen (e.g. 10 to about20 percent by weight) has been found to result in a substantiallyimproved process. More specifically, the resulting stabilized acrylicfibrous materials and films exhibit enhanced physical properties (i.e.strength and modulus), and the stabilization reaction is accelerated ina controlled manner in the substantial absence of undesirable fibercoalescence. The resulting stabilized fibrous material or film isnon-burning, and may be utilized as a fire resistant fiber, fabric, orfilm, or optionally carbonized or carbonized and graphitized to form acarbonaceous fibrous material or film.

BACKGROUND OF THE INVENTION In the past procedures have been proposedfor the conversion of fibers formed from acrylic polymers to a modifiedform possessing enhanced thermal stability. Such modification hasgenerally been accomplished by heating a fibrous material in anoxygen-containing atmosphere at a moderate temperature for an extendedperiod of time.

US. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda discloseprocesses for the conversion of fibers of acrylonitrile homopolymers orcopolymers to a heat resistant form. The stabilization of fibers ofacrylonitrile homopolymers and copolymers in an oxygen-containingatmosphere involves (1) a chain scission and oxidative cross-linkingreaction of adjoining molecules, (2) dehydrogenation reactions, as wellas (3) a cyclization reaction of pendant nitrile groups. It is generallyrecognized that the rate at which the stabilization reaction takes placeincreases with the temperature of the oxygen-containing atmosphere.However, the stabilization reaction must by necessity be conducted atrelatively low temperatures (i.e. below about 300 0.), since thecyclization reaction is exothermic in nature and must be controlled ifthe original fibrous configuration of the material undergoingst-abilization is to be preserved. Accordingly the stabilizationreaction tends to be time consuming, and economically demanding becauseof low productivity necessitated by the excessive time requirements.Prior processes proposed to shorten the period required by thestabilization reaction include that disclosed in US. Pat. No. 3,416,874.See also the processes of commonly assigned U.S. Ser. Nos. 777,- 902,filed Nov. 21, 1968, of K. H. Gump and D. E. Stuetz (now US. Pat. No.3,647,770) wherein a solution of an acrylic polymer containing a Lewisacid is heated e.g. at

3,813,219 Patented May 28, 1974 140 to 160 C.) to produce cyclization ofpendant nitrile groups, and the resulting solution is formed into acyclized acrylic fibrous material; 109,669, filed Jan. 25, 1971, of E.C. Chenevey and R. M. Kirnmel; and 200,184, filed Nov. 18, 1971, of K.H. Gump and D. E. Stuetz.

US. Pat. No. 3,242,000 to I. A. Lynch discloses an unrelated process forproducing carbonized textile products from acrylic textile productswherein a refractory metal oxide barrier coating is formed upon thesurface of fabric employing a heat treatment atmosphere which containsat least some oxygen (e.g. about 10 to about 20 percent oxygen).

While stabilized acrylic fibrous materials may be used directly inapplications where a non-burning fiber is required, demands for the samehave been increasingly presented by manufacturers of carbonized fibrousmaterials. Carbonized fibrous materials are commonly formed by heating astabilized acrylic fibrous material in an inert atmosphere, such asnitrogen or argon, at a more highly elevated temperature. During thecarbonization reaction elements such as nitrogen, oxygen, and hydrogenare substantially expelled. Accordingly, the term carbonized as used inthe art commonly designates a material consisting of at least aboutpercent carbon by weight, and generally at least about percent carbon byweight. Depending upon the conditions under which a carbonized fibrousmaterial is processed, it may or may not contain graphitic carbon asdetermined by the characteristic X-ray diffraction pattern of graphite.See, for instance, commonly assigned U.S. Ser. No. 777,275, filed Nov.20, 1968, of Charles M. Clarke (now abandoned) for a preferred procedurefor forming carbonized and graphitized fibrous materials from astabilized acrylic fibrous material.

It is an object of the invention to provide an improved process forforming thermally stabilized acrylic shaped articles.

It is an object of the invention to provide an improved process forforming fiame-proofed fibrous materials or films derived from acrylicpolymers.

It is an object of the invention to provide an improved process forforming thermally stabilized acrylic shaped articles wherein the thermalstabilization of an acrylic fibrous material or film is accelerated in acontrolled and non-deleterious manner.

It is an object of the invention to provide an improved process forforming thermally stabilized acrylic fibrous materials or films whichexhibit enhanced physical properties, i.e. strength and modulus.

It is an object of the invention to provide an improved process forforming thermally stabilized acrylic fibers which employs an enrichedoxygen-containing atmosphere on an expeditious basis in the absence ofexpected fiber coalescence.

It is another object of the invention to provide an improved process torforming thermally stabilized acrylic fibrous materials and films whereina superior product is produced which is suitable for carbonization, orcarbonization and graphitization.

These and other objects, as well as the scope, nature, and utilizationof the invention will be apparent from the following detaileddescription and appended claims.

SUMMARY OF THE INVENTION It has been found that an improved process forthe production of stabilized acrylic fibers and films exhibitingenhanced physical properties comprises:

(a) Providing a solution of (1) an acrylic polymer selected from thegroup consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers containing at least about 85 mol percent of acrylonitrileunits and up to about 15 mol percent of one or more monovinyl unitscopolymerized therewith wherein the pendant nitrile groups present inthe acrylic polymer are substantially uncyclized, (2) a minor quantityof stannous chloride, and (3) a solvent for the acrylic polymer and thestannous chloride,

(b) Forming from the solution an acrylic fibrous material orfilm whereinthe pendant nitrile groups present in the acrylic polymer aresubstantially uncyclized having incorporated therein about 0.5 topercent by weight of stannous chloride, and

(c) Heating the acrylic fibrous material or film having the stannouschloride incorporated therein in a gaseous atmosphere containing 30 to100 percent by weight molecular oxygen provided at a temperature ofabout 260 to 350 C. until a stabilized fibrous material or film isformed which retains its original configuration substantially intact andwhich is non-burning when subjected to an ordinary match flame.

DESCRIPTION OF PREFERRED EMBODIMENTS The acrylic polymer utilized as thestarting material is formed primarily of recurring acrylonitrile units.For instance, the acrylic polymer should generally contain not less thanabout 85 mol percent of acrylonitrile units and not more than about molpercent of units derived from a monovinyl compound which iscopolymerizable with acrylonitrile such as styrene, methyl acrylate,methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride,vinyl pyridine, and the like, or a plurality of such monomers. Thependant nitrile groups present within the acrylic precursor aresubstantially uncyclized.

The preferred acrylic precursor is an acrylonitrile homopolymer.Preferred acrylonitrile copolymers contain at least about 95 mol percentof acrylonitrile units and up to about 5 mol percent of one or moremonovinyl units copolymerized therewith.

The stannous chloride which is dissolved in the solvent for the acrylicpolymer may be either anhydrous [e.g. SnCl or hydrous [e.g. SnCl -2H O].When stannous chloride is present in the hydrous form, the water ofhydration is not included when calculating the concentration of stannouschloride for the purposes of the present specification and appendedclaims.

Suitable solvents which may be utilized in the present process arecapable of dissolving both the acrylic polymer and the stannouschloride. Representative organic solvents include N,N-dimethylformamide,N,N-dimethylacetamide, dimethyl sulfoxide, butyrolactone, andN-methyl-2-pyrrolidinone. The preferred solvents are those which arecommonly utilized during the spinning of fibers of acrylonitrilehomopolymers and copolymers. The particularly preferred solvents areN,N-dimethylformamide and N,N dimethylacetamide. The solvent may bedried by running through a bed of drying agent (e.g. Linde sieves,activated alumina, etc.), and moisture excluded by blanketing with dryair or nitrogen.

The concentration of the acrylic polymer in the solvent may be variedwidely, e.g. about 5 to about 30 percent by weight based upon the weightof the solvent. Preferred concentrations range from 10 to 25 percentacrylic polymer by weight based upont the weight of the solvent.

The stannous chloride is present in the solution of acrylic polymer in aminor concentration, i.e. about 0.5 to percent by weight based upon theweight of the acrylic polymer. In a preferred embodiment of the processthe stabilization promoting agent is present in a concentration of about1 to 10 percent by weight based upon the weight of the acrylic polymer.

In a preferred embodiment of the process employing anN,N-dimethylacetamide solvent the solution of acrylic polymer andstannous chloride additionally contains 0.1 to 5.0 percent by weightbased upon the total weight of the solution (0.5 to 2.0 percent in aparticularly preferred embodiment) of lithium chloride dissolvedtherein. The incorporation of lithium chloride serves the function oflowering and preserving upon standing the viscosity of the solution. Thedesired solution fluidity and mobility for spinning or casting areaccordingly efiiciently maintained even upon the passage of time.

The solution of the acrylic polymer and stannous chloride suitable forextrusion may be formed by any convenient technique. For instance, theacrylic polymer while in particulate form together with stannouschloride may be added to the solvent with stirring while maintained atabout 50 to C. It is recommended that any heating of the solution inexcess of about C. be of limited duration, i.e. no more than a fewminutes, so that no substantial degree of cyclization of pendant nitrilegroups within the acrylic polymer occurs while dissolved in the solvent.

The solution is preferably filtered, such as by passage through a plateand frame press provided with an appropriate filtration medium, prior toforming a fibrous material or film.

The solution containing the acrylic polymer and the stannous chloride ispreferably converted into a fiber or film through the substantialelimination of the solvent following extrusion through a shaped orificeemploying conventional solution spinning techniques (i.e. by dryspinning or wet spinning). As is known in the art, dry spinning iscommonly conducted by passing the solution through an opening ofpredetermined shape into an evaporative atmosphere (e.g. nitrogen) inwhich much of the solvent is evaporated. Wet spinning is commonlyconducted by passing the solution through an opening of predeterminedshape into a suitable coagulation bath. Acrylic films may also be formedby casting wherein a layer of the solution is placed upon a support andthe solvent evaporated.

When wet spinning is utilized in the fiber or film forming step of theprocess, a coagulation bath is selected which is capable of preservingthe requisite catalytic quantity of stannous chloride within theresulting fibrous material or film. More specifically, the bathpreferably exhibits no propensity to leach out and dissolve the stannouschloride below the minimum level required for catalysis during thesubsequent heat treatment step (described hereafter). Such coagulationbath may inherently possess no substantial tendency to dissolve thestannous chloride. Alternatively, the coagulation bath which is selectedmay have its inherent tendency to dissolve stannous chloride diminishedby preliminarily dissolving a substantial quantity of stannous chloride,or other compound therein. The coagulation bath is preferablysubstantially anhydrous. A preferred wet spinning technique is disclosedin commonly assigned U.S. Ser. No. 28,545, filed Apr. 14, 1970 (now US.Pat. No. 3,657,409), which is herein incorporated by reference.

The shaped orifice or spinneret utilized during the extrusion maycontain a single hole through which a single filament is extruded, andpreferably contains a plurality of holes whereby a plurality offilaments may be simultaneously extruded in yarn form. The spinneretpreferably contains holes having a diameter of about 50 to microns whenproducing relatively low denier fibers having an as-spun denier of about8 to 24 denier per filament. Alternatively, acrylic films of relativelythin thickness, e.g. about 1 to 10 mils, may be formed, when theextrusion orifice is a rectangular slit. Generally stated, the solutionmay be formed into an acrylic fibrous material or film having stannouschloride incorporated therein utilizing conventional fiber or filmforming techniques with a minor quantity of the stannous chloride beingmerely added to the polymer dope.

The resulting as-spun fibrous material or film is preferably maintainedin a continuous length configuration throughout the process. At anintermediate point prior to heat treatment the fibrous material mayalternatively be transformed into another fibrous assemblage, e.g. atow, fabric, or yarn of greater total denier.

When the fibrous material is a continuous multifilament yarn, a twistmay be imparted to the same to improve the handling characteristics. Forinstance, a twist of about 0.1 to 5 t.p.i., and preferably about 0.3 to1.0, t.p.i. may be utilized. Also a false twist may be used instead ofor in addition to a real twist. Alternatively, one may select bundles offibrous material which possess substantially no twist.

The fibrous material may be drawn in accordance with conventionaltechniques in order to improve its orientation. For instance, thefibrous material may be drawn by stretching while in contact with a hotshoe at a temperature of about 140 to 160 C. Additional representativedrawing techniques are disclosed in US. Pat. Nos. 2,455,- 173;2,948,581; and 3,122,412. It is recommended that fibrous materials priorto the heat treatment (described hereafter) be drawn to a singlefilament tenacity of at least about 3 grams per denier. If desired,however, the fibrous material may be more highly oriented, e.g. drawn upto a single filament tenacity of about 7.5 to 8 grams per denier, ormore. Additionally, the acrylic films optionally may be eitheruniaxially or biaxially oriented prior to the heat treatment describedhereafter.

Immediately prior to the heat treatment step employing a gaseousatmosphere of enriched molecular oxygen content the acrylic fibrousmaterial or film contains stannous chloride substantially uniformlyincorporated therein in a concentration of about 0.5 to 10 percent byweight, and preferably in a concentration of 1 to 5 percent by weight.

The resulting acrylic fibrous material or film containing stannouschloride incorporated therein is heated in a gaseous atmospherecontaining 30 to 100 percent by weight molecular oxygen provided at atemperature of about 260 to 350 C. until a stabilized fibrous product orfilm is formed which retains its original configuration substantiallyintact and which is non-burning when subjected to an ordinary matchflame. The portion of the gaseous atmosphere other than molecularoxygen, if any, is preferably substantially unreactive with the acrylicfibrous material or film during the stabilization treatment, e.g. it mayinclude nitrogen, hydrogen, carbon dioxide, carbon monoxide, argon,helium, etc. In a preferred embodiment of the process, theoxygen-containing atmosphere is air enriched with molecular oxygen.Molecular oxygen is preferably present in the gaseous atmosphere in aconcentration of 35 to 100 percent by weight, and most preferably in aconcentration of about 40 to 60 percent by weight. Preferredtemperatures for the oxygen-containing atmosphere range from about 290to 310 C. If desired, the fibrous material or film may be exposed to atemperature gradient wherein the temperature is progressively increased.The presence of an enriched oxygen atmosphere in combination with thepresence of stannous chloride has been found to be of prime importancein accomplishing the improved stabilization results discussed hereafter.

For best results during the stabilization reaction uniform contact withthe gaseous atmosphere throughout all portions of the stannous chloridecontaining acrylic material is encouraged. Such uniform reactionconditions can best be accomplished by limiting the mass of fibrousmaterial or film at any one location so that heat dissipation fromwithin the interior of the same is not unduly impaired, and free accessto molecular oxygen is provided. For instance, the acrylic fibrousmaterial or film may be placed in the gaseous atmosphere While woundupon a support to a limited thickness. In a preferred embodiment of theinvention, the stannous chloride containing acrylic fibrous material orfilm is continuously passed in the direction of its length through theheated gaseous atmosphere. For instance, a continuous length of theacrylic fibrous material or film may be passed through a circulatingoven or the tube of a muflle furnace. The speed of passage through theheated oxygen-containing atmosphere will be determined by the size ofthe heating zone and the desired residence time.

The period of time required to complete the stabilization reactionwithin the gaseous atmosphere is generally inversely related to thetemperature of the atmosphere, and is also influenced by the denier ofthe acrylic fibrous material or the thickness of the film undergoingtreatment, and the concentration of molecular oxygen in the atmosphere.Treatment times in the oxygen-containing atmosphere accordingly commonlyrange from about 6 minutes to 60 minutes.

Thhe stabilized acrylic fibrous materials or films formed in accordancewith the present process are black in appearance, retain substantiallythe same configuration as the starting material, are non-burning whensubjected to an ordinary match flame, commonly have a bound oxygencontent of at least 7 (e.g. 7 to 12) percent by weight as determined bythe Unterzaucher, or other suitable analysis, commonly contain fromabout 50 to 65 percent carbon by weight, and commonly contain about 0.4to 8 percent tin by weight.

The theory whereby the presence of stannous chloride in combination witha greater than usual oxygen concentration in the gaseous atmosphereproduces improved stabilization results is considered complex andincapable of simple explanation. The results achieved are considered tobe surprising and unexpected. While it has been suggested in the pastthat acrylic stabilization reactions can be conducted in an atmosphereof air enriched with oxygen, the results of such stabilizationconditions have tended to be less than optimum particularly itrelatively high stabilization temperatures (e.g. 260 C. and above) areselected because of the increased tendency for an explosive exotherm tooccur under such conditions. Such an exothermic reaction at the veryleast produces a weak and brittle product and may result in a completebreakage of the acrylic fiber or fragmentation of the acrylic film. Ithas now been found after extensive experimentation that While stannouschloride has the ability to accelerate the kinetics of the cyclizationportion of the stabilization reaction in air, that the presence of thiscompound has the concomitant tendency to retard the dehydrogenation andoxidative cross-linking portions of the stabilization reaction.Additionally, even when the oxygen concentration of the gaseousstabilization atmosphere is increased, the mechanical properties of theresulting product are surprisingly not diminished and even moresurprisingly are enhanced. For instance, stabilized products formed inthe present process in the presence of stannous chloride exhibit ahigher tenacity and modulus than if produced in the presence of stannouschloride in air. Not only is the stabilization reaction accelerated, butno substantial fiber coalescence occurs and less fiber weight lossresults. The process of the present invention proceeds at an expeditiousrate in a controlled fashion with the fiber temperature during thestabilization reaction more closely approximating that of the gaseousatmosphere while eliminating a deleterious exothermic reaction.

In our commonly assigned Ser. No. 248,372, filed concurrently herewith,is disclosed a related process wherein stannous chloride is incorporatedinto a previously formed acrylic fibrous material or film by contactwith a solution of the same provided at a moderate temperature, and theresulting stannous chloride impregnated fiber or film stabilized in agaseous atmosphere containing more than the usual concentration ofoxygen.

The stabilized fibrous material resulting from the stabilizationtreatment of the present process is suitable for use in applicationswhere a fire resistant fibrous material is required. For instance,non-burning fabrics may be formed from the same. As previouslyindicated, the stabilized acrylic fibrous materials are particularlysuited for use as intermediates in the production of carbonized fibrousmaterials. Such amorphous carbon or graphitic carbon fibrous productsmay be incorporated in a binder or matrix and serve as a reinforcingmedium. The carbon fibers may accordingly serve as a lightweight loadbear- 7 ing component in high performance composite structures whichfind particular utility in the aerospace industry.

The stabilized film resulting from the stabilization treatment issuitable for use in applications where a fire resistant sheet materialis required. Such stabilized films may also be utilized as intermediatesin the production of carbonized films. Carbonized films may be utilizedin the formation of lightweight high temperature resistant laminateswhen incorporated in a matrix material (e.g. an epoxy resin).

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the examples.

EXAMPLE I A solution of an acrylic polymer is formed while employingN,N-dimethylformamide as a solvent while maintained at 50 C. Particulateacrylonitrile homopolymer is added to the solvent with stirring in aconcentration of 25 percent by weight based upon the weight ofN,N-dimethylformamide. Stannous chloride is dissolved in the acrylicpolymer solution with stirring in a concentration of 7.5 percent 'byweight based upon the weight of the acrylic polymer. The pendant nitrilegroups of the acrylic polymer while dissolved in theN,N-dimethylformamide are substantially uncyclized.

Following filtration the solution is fed while at a temperature of 35 C.to a standard cup type spinneret having a circle of 40 holes each havinga diameter of 50 microns. The solution is extruded through the spinneretinto a coagulation bath of approximately 60 parts by weight of ethyleneglycol and approximately 40 parts by weight of N,N-dimethylformamideprovided at 25 C. to form a continuous length of acrylic yarn havingstannous chloride substantially uniformly incorporated therein. Theresulting yarn fibers possess a denier per filament of about 4, aresubsequently washed in water to remove residual solvent, are dried, andare drawn at a draw ratio of about :1 by passage over a hot shoe at atemperature of about 140 C. to produce an acrylic fibrous materialexhibiting a single filament tenacity of about 4.5 grams per denier. Thependant nitrile groups of the resulting acrylic fibrous material aresubstantially uncyclized.

A portion of the yarn containing 5 percent by weight stannous chlorideis next stabilized on a continuous basis by heating in a circulatinggaseous atmosphere of air encircled with molecular oxygen provided in amuffle furnace. The total oxygen concentration in the atmosphere is 40percent by weight. The gaseous atmosphere is provided at a temperatureof 300 C. and the residence time therein is 8 minutes. The yarn ismaintained under a longitudinal tension sufiicient to maintain asubstantially constant length during the stabilization reaction.

The resulting stabilized yarn is black in appearance, non-brittle,flexible, has a textile-like hand, retains its original fibrousconfiguration substantially intact, is nonburning when subjected to anordinary match flame, retains strength after glowing in a match flame,and has an oxygen content in excess of 8 percent by weight as determinedby the Unterzaucher analysis.

In a control run a sample of acrylonitrile homopolymer yarn is passedthrough the muflie furnace in an identical manner with the exceptionthat the yarn was formed from a N,N-dimethylformamide solution whichcontains no stannous chloride. The resulting yarn is coalesced,extremely brittle, and non-flexible.

The resulting stabilized yarn of Example -I is carbonized andgraphitized in accordance with the teachings of US. Ser. No. 777,275,filed Nov. 20, 1968, of Charles M. Clarke (now abandoned) which isherein incorporated by reference. The resulting graphite yarn exhibitssatisfactory tensile properties.

8 EXAMPLE 11 Example I is repeated with the exception that portions ofthe stannous chloride containing yarn are stabilized for 15 minutes inair enriched with molecular oxygen atmospheres containing 40 and percentby weight molecular oxygen by weight provided at 290 C.

In each instance the resulting stabilized yarn is black in appearance,non-brittle, flexible, has a textile-like hand, retains its originalfibrous configuration substantially intact, is non-burning whensubjected to an ordinary match flame, retains strength after glowing ina match flame, and has an oxygen content in excess of 8.5 percent byweight as determined by the Unterzaucher analysis.

The stabilized yarn produced in the 40 percent by weight molecularoxygen gaseous atmosphere possesses a single filament tenacity of 2.5grams per denier, and a Youngs modulus of 300 grams per denier.

The stabilized yarn produced in the 80 percent by weight molecularoxygen gaseous atmosphere possesses a single filament tenacity of 2.5grams per denier, and a Youngs modulus of 350 grams per denier.

For comparative purposes the process of Example II is repeated with theexception that the gaseous atmosphere is air only and contains 20.9percent by weight molecular oxygen. The resulting fibers burn whensubjected to an ordinary match flame, exhibit a single filament tenacityof only 0.5 gram per denier, and a single filament Youngs modulus ofonly 150 grams per denier.

EXAMPLE I11 Example I is repeated with the exception that a portion ofthe solution of the acrylonitrile homopolymer containing stannouschloride dissolved therein was placed upon a glass support in athickness of 10 mils and the solvent evaporated in a circulating airoven providedat 110 C. to form a flexible film. The pendant nitrilegroups of the acrylic polymer of the resulting film are substantiallyuncyclized. Stannous chloride is substantially uniformly incorporatedwithin the film in a concentration of approximately 7.5 percent byweight. The resulting stannous chloride containing film is nextsuspended for 7 minutes in a circulating air enriched with molecularoxygen atmosphere containing 40 percent by weight molecular oxygenprovided in at 300 C. wherein it is converted to a stabilized form whileretaining its original configuration substantially intact. The resultingstabilized film is black in appearance, non-brittle, flexible,non-burning when subjected to an ordinary match flame, and contains abound oxygen content in excess of about 7 percent by weight asdetermined by the Unterzaucher analysis.

In a control run a sample of an acrylonitrile homopolymer film isprocessed in an identical manner with the exception that the film isformed from a N,N-dimethylformamide solution which contains no stannouschloride. The resulting film is brittle and falls apart.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in theart. Such variations andmodifications are to be considered within the purview and scope of theclaims appended hereto.

We claim:

1. An improved process for the production of thermally stabilizedacrylic fibers and films exhibiting enhanced physical propertiescomprising:

(a) providing a solution of (1) an acrylic polymer selected from thegroup consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers containing at least about mol percent of acrylonitrile unitsand up to about 15 mol percent of one or more monovinyl unitscopolymerized therewith wherein the pendant nitrile groups present insaid acrylic polymer are substantially uncyclized, (2) a minor quantityof stannous chloride, and (3) a solvent for said acrylic polymer andsaid stannous chloride, (b) forming from said solution an acrylicfibrous material or film wherein the pendant nitrile groups present insaid acrylic polymer are substantially uncyclized having incorporatedtherein about 0.5 to

10 percent by weight of said stannous chloride based upon the weight ofsaid acrylic polymer, and (c) heating said acrylic fibrous material orfilm having said stannous chloride incorporated therein in a gaseousatmosphere containing 30 to 100 percent by weight molecular oxygenprovided at a temperature of about 260 to 350 C. until a thermallystabilized fibrous material or film is formed which is black inapearance, retains its original configuration substantially intact,contains a bound oxygen content of at least 7 percent by weight, andwhich is non-burning when subjected to an ordinary match flame, with anyportion of said gaseous atmosphere other than molecular oxygen beingsubstantially unreactive with the material undergoing stabilization.

2. An improved process of claim 1 in which said acrylic polymer presentin said solution is an acrylonitrile homopolymer.

3. An improved process of claim 1 in which said acrylic polymer presentin said solution is an acrylonitrile copolymer containing at least about95 mol percent of acrylonitrile units and up to about mol percent of oneor more monovinyl units copolymerized therewith.

4. An improved process of claim 1 in which said acrylic polymer ispresent in said solution in a concentration of about 5 to 30 percent byweight based upon the weight of the solvent.

5. An improved process of claim 1 in which said stannous chloride ispresent in said solution in a concentration of about 0.5 to 20 percentby weight based upon the weight of said acrylic polymer.

6. An improved process of claim 1 in which said stannous chloride ispresent in said solution in a concentration of about 1 to percent byweight based upon the weight of said acrylic polymer.

7. An improved process of claim 1 in Which said solvent for said acrylicpolymer and said stannous chloride is selected from the group consistingof N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide,butyrolactone, and N-methyl-Z-pyrrolidinone.

8. An improved process of claim 1 in which said solution is extrudedthrough a shaped orifice to form a fibrous material.

9. An improved process of claim 8 in which said fibrous material isdrawn to a single filament tenacity of at least about 3 grams per denierprior to heating in said gaseous atmosphere containing 30 to 100 percentby weight molecular oxygen.

10. An improved process of claim 9 in which said acrylic fibrousmaterial is a continuous multifilament yarn.

11. An improved process of claim 1 in which said solution is extrudedthrough a shaped orifice to form a film.

12. An improved process of claim 1 in which said fibrous material orfilm contains said stannous chloride in a concentration of about 1 to 5percent by wegiht based upon the weight of said acrylic polymerimmediately prior to heating said gaseous atmosphere containing 30 to100 percent molecular oxygen by weight.

13. An improved process of claim 1 in which said gaseous atmospherecontains about 35 to 100 percent molecular oxygen by weight.

14. An improved process for the production of thermally stabilizedacrylic fibers exhibiting enhanced physical properties comprising:

(a) providing a solution of (1) an acrylic polymer selected from thegroup consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers containing at least about mol percent of acrylonitrile unitsand up to about 15 mol percent of one or more monovinyl unitscopolymerized therewith wherein the pendant nitrile groups of saidacrylic polymer are substantially uncyclized, (2) a minor quantity ofstannous chloride, and (3) a solvent for said acrylic polymer and saidstannous chloride,

(b) extruding said solution through a shaped orifice to form an acrylicfibrous material wherein the pendant nitrile groups present in saidacrylic polymer are substantially uncyclized having stannous chlorideincorporated therein,

(c) drawing said acrylic fibrous material to a single filament tenacityof at least about 3 grams per denier,

(d) heating said acrylic fibrous material having incorporated thereinabout 1 to 5 percent by weight stannous chloride based upon the weightof said acrylic polymer in a gaseous atmosphere containing 35 to 100percent molecular oxygen provided at a temperature of about 290 to 310C. until a thermally stabilized fibrous material is formed which isblack in appearance, retains its original configuration substantiallyintact, contains a bound oxygen content of at least 7 percent by weight,and which is nonburning when subjected to an ordinary match flame, withany portion of said gaseous atmosphere other than molecular oxygen beingsubstantially unreactive with the material undergoing stabilization.

15. An improved process of claim 14 in which said acrylic polymer is anacrylonitrile homopolymer.

16. An improved process of claim 14 in which said acrylic polymer is anacrylonitrile copolymer containing at least about mol percent ofacrylonitrile units and up to about 5 mol percent of one or moremonovinyl units copolymerized therewith.

17. An improved process of claim 14 in which said solvent for saidacrylic polymer and said stannous chloride is selected from the groupconsisting of N,N- dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, butyrolactone, and N-methyl-Z-pyrrolidinone.

18. An improved process of claim 14 in which said acrylic fibrousmaterial is a continuous multifilament yarn.

19. An improved process of claim 14 in which said gaseous atmospherecontains about 40 to 60 percent molecular oxygen by weight.

References Cited UNITED STATES PATENTS 3,647,770 3/1972 Gump et al.423-447 X 3,539,295 11/1970 Ram 8-115.5 X 3,427,120 2/1969 Shindo et al.8115.5 3,242,000 3/ 1966 Lynch 8Acrylo 3,729,549 4/1973 Gump et al.8115.5

LEON D. ROSDOL, Primary Examiner H. WOLMAN, Assistant Examiner US. Cl.X.R. 423-447

